Parameters influencing ATmax

The metastable zone width is sensitive to several factors.

1) Heterogeneous nucleation:

We can compare the value of homogeneous nucleation r * with the value of heterogeneous nucleation r* = l* /2 for a spherical nucleus, of which fraction a of the surface is in contact with a solid material (ceramic or metal). Accordingly, we take k ш = n/6 and ka =n

We observe that the critical size for heterogeneous nucleation is lower. In other words, the impurities in the suspension benefit nucleation. For an impeccably pure solution, ATmax is between 10 and 20°C, while for an untreated solution, ATmax does not exceed 2-3°C.

2) Concentration fluctuations:

These fluctuations lead to the apparition of crystal embryos of critical size. They are encouraged by:

  • - low viscosity;
  • - mechanical action (collisions, ultrasound);
  • - high concentration c* at equilibrium.

In these conditions, ATmax decreases.

3) The chemical combination model:

We can consider the formation of critical cores as the end result of a series of reactions involving crystal embryos Ei containing i molecules.

We understand that this series of reactions is hindered:

  • - by the addition to the solution of an inorganic compound complexing which blocks crystallizable molecules. Thus, polyphosphates hinder the scaling of boiler tubes;
  • - if, for crystallization, each molecule must carry with it a high number of hydration water molecules (more generally, solvation molecules);

- if the solution is subjected to prior heating over a period of one, two or three hours at a temperature greater than the saturation temperature by 10-60°C. Essentially, this shifts the precedent reactions to the left.

 
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